1. Field of the Invention
The present invention relates generally to the field of internal combustion engine control systems and, in particular, to those closed loop control systems as are responsive to various engine operational parameters for controlling the ignition of internal combustion engines. In greater detail, the present invention is concerned with a closed loop ignition control system that is responsive to various parameters such as engine speed, crankshaft position, load as indicated by the manifold vacuum, and engine cylinder position.
2 Description of the Prior Art
Electronic ignition and fuel control systems for internal combustion engines are finding acceptance in the automotive and allied industries as rigid efficiency and pollution standards are imposed by the government. The first generation of these electronic controls were open loop systems which became progressively complex as the standards were raised. The number of variables needed to be detected as well as auxiliary circuits for providing corrections for these variables increased with each raising of the standards. From the conception of electronic control systems for internal combustion engines, it has been known that if the control systems could be closed about the engine, simpler control techniques could be developed. This would reduce the number of variables needed to be detected, reduce the complexity of the control systems, and at the same time improve the overall efficiency.
K. W. Randall and J. D. Powell from Stanford University in their research under a Department of Transportation sponsored project determined that for maximum efficiency of an internal combustion engine, the spark timing should be adjusted to provide a maximum cylinder pressure at a crankshaft angle 15.degree. after the piston's top dead center (TDC) position. The results of this investigation are published in a Final Report No SUDAAR503 entitled "Closed Loop Control of Internal Combustion Engine Efficiency and Exhaust Emission". The report contains a block diagram of a closed loop system in which a sensor detects the angle at which peak pressure occurs and then compares this measured angle with the desired 15.degree. angle. An error signal, generated when the measured angle differs from the desired angle, is used to correct the ignition timing signal generated, regardless of other sensed engine parameters.
J. Glaser and J. D. Powell describe in a subsequent report SAE 810058 their investigation of a closed loop control system for retarding and/or advancing ignition firing as a function of maximum cylinder pressure. The authors Glaser and Powell found that peak cylinder pressure did not decrease for settings of spark advance less than 10.degree. before top dead center (BTDC) and, as a result, peak cylinder pressure measurements could not be used for a feedback control system of this type at relatively low speeds. Typically in such closed loop ignition control systems, relatively small spark advances are set to achieve low speeds. At higher speeds and, thus, higher spark advance angles, it appeared that cylinder pressure measurements could be used to effect a feedback type of control.
U.S. Pat. No. 4,002,155 of Harned teaches a closed loop ignition system in which engine knock-induced vibrations are are detected by an engine mounted accelerometer. The system counts the number of individual ringing vibrations that occur in a predetermined angular rotation of the crankshaft. When the number of ringing vibrations exceeds a first predetermined number, the engine spark timing is retarded and when the number of ring vibrations is less than a second predetermined number, the spark timing is advanced. Such a closed loop ignition system is applicable to engine control under load and at high engine speeds and is limited by the noise normally associated with the closing and opening of the engine valves and cylinder movement. Thus, the Harned et al system would not be particularly applicable to a closed loop controlled ignition system for setting the spark advance at low or idle engine speeds.
U.S. Pat. No. 3,897,766 of Pratt, Jr. et al. discloses a torque sensor which measures the twist in the output shaft of the prime mover to measure the torque. The measured torque and engine speed are used to close the loop about the engine. Such torque measurements are used to calculate horsepower by multiplying the torque by engine speed to optimize ignition control. It is apparent that such torque sensors could not be used for ignition control at engine idling speeds.
U.S. Pat. No. 4,026,251 of Schweitzer et al. describes a closed loop ignition control system that moves the control back and forth, i.e. dithering, to obtain an optimum value of spark angle. The dithering of an engine requires a relatively long period of time that would limit the usefulness of such a technique to motor control at stabilized, relatively high engine speeds.
Various types of closed loop fuel control systems for internal combustion engines have been developed in which the loop is closed about different engine parameters. One of the parameters about which the loop is closed is the composition of the exhaust gas as taught by Seitz in U.S. Pat. No. 3,815,561. The system taught by Seitz uses an oxygen (O.sub.2) sensor detecting the concentration of oxygen in the exhaust gas and closes the loop about a stoichiometric mixture of air and fuel. However, a stoichiometric mixture of air and fuel has been found to be too rich for very efficient operation of the engine. Various techniques have been employed to operate the engine at leaner air/fuel (A/F) ratios but the ability to achieve reliable closed loop control at the desired leaner A/F mixture is limited by the characteristics of available oxygen sensors. In order to reduce the A/F ratios the Seitz closed loop system employs a class zero servo action to control engine ignition whereby the engine oscillates about the correct stoichiometric mixture, which means that the controlled engine A/F ratio will oscillate about this parameter.
U.S. Pat. No. 3,789,816 of Taplin et al. measures engine roughness as the parameter about which the loop is closed. In this system, the A/F mixture is leaned out until a predetermined level of engine roughness is achieved. The magnitude of engine roughness is selected to correspond with a level of engine roughness at which the A/F mixture is made as lean as possible to the point that the formation of such exhaust gas as HC and CO is minimized without the drivability of the particular vehicle being unacceptable. Engine roughness as measured in the Taplin et al patent is the incremental change in the rotational velocity of the engine's output as a result of the individual torque impulses received from each of the engine's cylinders. The closing of the fuel control loop about engine roughness appears to be the most effective means for maximizing the fuel efficiency of the engine.
U.S. Pat. No. 4,044,236 of Bianchi et al. teaches measuring the rotational periods of the crankshaft between two sequential revolutions of the engine. The differential is digitally measured in an up-down counter counting at a frequency proportional to the engine speed.
U.S. Pat. No. 4,044,234 of Frobenius et al. teaches measuring the rotational periods of two equal angular intervals, one before and one after the top dead center position of each cylinder position. The difference between the two rotational periods for the same revolution is compared against a particular reference value and an error signal is generated when the change exceeds the reference value. Frobenius in U.S. Pat. No. 4,044,235 teaches an alternate roughness control system wherein the periods of three sequential revolutions are compared to determine engine smoothness. The above discussion reflects various ways in which engine roughness as detected by various means including the variations in the rotational velocity of the flywheel is used to close the loop about the engine.
U.S. Pat. No. 3,957,023 of Peterson discloses a closed loop ignition control system that measures the occurrence of peak cylinder pressure and uses that parameter to adjust the spark angle so that the peak cylinder pressure occurs on the next cycle at the optimum angular position to obtain the best mean torque output from the engine under control. However, Peterson did not provide means operative at lower engine speeds that are capable of responding rapidly to adjust the spark firing angle so that his closed loop control would be stable. Instead, Peterson suggested that his system would produce spark ignition signal 180.degree. out of phase that would cause engine speed oscillation.
U.S. Pat. No. 4,197,767 of Leung discloses a closed loop engine control system that measures the engine's temperature and the uniformity of crankshaft rotation to vary spark advance and fuel delivery. It is understood that when a load is applied to the engine that the period or time of crankshaft rotation will gradually increase. Leung employs sensor pick-ups disposed to sense the rotation of the engines crankshaft, measuring the output of the sensors to determine the time of each crankshaft rotation and the difference between the time intervals of rotation of successive rotations of the engine crankshaft, and uses such differences to control spark advance and fuel delivery. However, the Leung systems employs a flywheel coupled to its engine, that significantly reduces speed perturbations at medium and higher speeds, i.e. those speeds above 1500 RPM. In particular, the speed perturbations or differences between successive revolutions of the engine are damped out and, thus, the Leung closed loop engine control is unable to effect control based upon the imposition of engine load and the rotational speed changes.
Rotational forces are derived from engines by the igniting of air/fuel mixtures injected into cylinders, to impart rectilinear movement to pistons disposed within the cylinders of the engine, whereby rotational forces are imparted to a crankshaft. Spark plugs are disposed within each cylinder and are energized to create a spark igniting the fuel mixture; the spark is timed with respect to the TDC position of the crankshaft to cause burning of the A/F mixture to impart forces on the cylinder and, therefore, on the crankshaft at a point in time after the cylinder has reached its TDC position. The angular position of the crankshaft is typically measured with respect to the TDC position of the cylinder. In particular, the spark is generated at a point in time with respect to the angular position of the engines crankshaft; typically, the spark is generated at a position before the TDC position to ensure that the A/F mixture will be ignited and that the A/F mixture burning will take place at a point in time after the piston reaches its TDC position. The angular position of the crankshaft at the point in time that the spark is generated is commonly known as the spark advance angle .theta.a and is measured in reference to the TDC position. Because the spark advance angle .theta.a directly effects when the burning of the gas-air mixture takes place, the spark advance angle .theta.a also effects the amount of torque that will be delivered to the crankshaft. The relationship between the spark advance angle .theta.a and the crankshaft torque is a first order function and must be controlled precisely to obtain maximum fuel economy and to minimize the pollutants emitted by the engine.
Typically, engines of the prior art employ distributors rotatively coupled to the engine to close a set of switches typically known as "points" to apply a high voltage to energize the spark plugs, thus generating a spark to ignite the A/F mixtures. More specifically, the distributors include a cam that is rotatively coupled to the engine and disposed to contact the points, whereby a circuit to a corresponding spark plug is completed. The physical position of the points and thus the spark advance angle .theta.a could be adjusted by a mechanical device in the nature of a governor to adjust the spark advance angle .theta.a as a function of the engine speed. Typically, the spark advance angle .theta.a is increased as the engine speed increases. In addition, prior art devices have employed sensors disposed to measure the manifold vacuum or pressure to provide an indication of the mechanical load imposed upon the engine. At no load, the manifold vacuum is high; when a load is applied to the engine, the manifold vacuum decreases. Typically such manifold vacuum measuring devices employed a vacuum diaphragm, that is mechanically coupled to the distributor, whereby the position of the distributor points and thus the spark advance angle may be adjusted as a function of manifold vacuum and thus engine load. Because such mechanical devices are limited in terms of accuracy and the degree to which they be controlled, electronic controls and, in particular, closed looped ignition systems have been employed to increase fuel efficiency and to decrease pollution emission.
The U.S. Government has imposed increasingly strict controls on both pollution and fuel efficiency standards for gasoline fueled engines. It is readily appreciated that such standards are mutually exclusive in that as steps are taken to increase the fuel efficiency, it becomes increasingly difficult to maintain the levels of pollution emission. Such pollution controls and fuel efficiency standards are expressed respectively in terms of the weight of emitted pollutants (grams) per mile of vehicle travel and in miles per gallon, without consideration of vehicle weight or engine performance. Thus, auto manufacturers are required to meet these control and standards to manufacture lighter, but not necessarily safer vehicles.
As a result, vehicle performance has suffered. Vehicle performance may be considered as (1) the ability of an engine to delivery an optimum mean torque to its crankshaft and (2) the ability to run smoothly at all speeds and, in particular, to run smoothly at idle speed during warm-up periods. U.S. Government controls and standards require exhaustive vehicle testing under varying conditions as defined in a EPA's "Federal Urban Driving Test". This test requires that vehicle engine emit limited pollutants during its warm-up period; without pollution control devices, some engines emit over one half of their total pollutants during this warm-up period. To meet such EPA tests, many emission control systems retard the spark advance angle .theta.a during critical portions of the EPA such as during warm-up. A critical region, known as the EPA pocket, occurs at lower engine speeds while driving the urban "driving" cycle. Typically, emission control systems retard the spark advance angle .theta.a thus limiting pollution emission, but at the expense of good engine performance. In particular, the spark advance angle .theta.a is advanced as a non-linear slope function of engine speed. The mechanical devices of the prior art, as well as many of the electronic controls, are able to implement such a function of spark advance angle .theta.a versus engine speed linearly, but with relatively poor accuracy and limited adjustment and, in particular, that the engine can not be adequately timed to meet the new tough EPA standards.
In the prior art, manifold vacuum sensors have been used to sense engine load and, in particular, the manifold vacuum and to use that parameter to adjust, i.e. retard, the spark advance angle .theta.a as a function of increasing engine loads to prevent detonation. However to retard the spark advance angle .theta.a, it is first necessary to advance the spark advance angle .theta.a when no load is placed upon the engine. However, this presents a problem at idle speeds in the order of 800 RPM, where the engine generally has no load imposed thereon and thus a high manifold vacuum. To solve this problem, mechanically implemented manifold vacuum sensors are coupled to a "ported" vacuum by connecting the vacuum line from a mechanical diaphragm to the manifold vacuum sensing above the throttle plate of the carburator, which prevents spark advance at idle speeds when the throttle plate is closed.
U.S. Pat. No. 4,015,566 of Walh discloses an electronic ignition system for an internal combustion engine that controls the timing of the ignition instant with respect to the measured crankshaft position as function of engine speed. In particular, the Walh system employs a transducer for providing a first train of pulses, one pulse for each revolution of the crankshaft, and a second train of speed pulses. The first and second trains of pulses are applied to a digital/analog converter with which includes an electronic counter. The output of the digital/analog converter is apparently indicative of the crankshaft position and is applied to a comparator. The second train of speed pulses is applied to a speed-voltage converter whose output as representative of crankshaft or engine rotational speed is applied to a function generator which generates a signal representative of ignition timing dependent upon the sensed crankshaft rotation to be applied to the comparator, which compares the two input signals and upon coincident, applies a signal to the engine distributor to effect ignition control.
As described above, ignition control is effected by setting the ignition instant with respect to the TDC position to define the degree of the spark advance angle .theta.a therewith. Considering the Wahl and like systems, the crankshaft position transducer or sensor may be coupled to the distributor shaft which is geared down by a ratio of 2:1 with respect to the crankshaft of a four cycle Otto engine. For such a four cylinder engine, such the crankshaft position transducer would output four pulses (of the first train) for each revolution of the distributor shaft and two pulse for each revolution of the crankshaft. At low speeds in the order of 600 RPM, the crankshaft revolves only 10 times per second. At 6000 RPM, the crankshaft revolves 100 times per second. Thus at 600 RPM, the transducer would output two such speed pulses per crankshaft revolution or one pulse every 1/20 of a second. In practice, the ignition instant occurs within an arc of the crankshaft rotation from a 45.degree. BTDC position to its TDC position. As a practical matter, data indicative of this arc of crankshaft rotation is the only data of interest for spark advance timing. In such systems as described by Walh, the crankshaft position information is dependent upon the manner in which the second train of speed pulses is accessed and measured. The Walh system is typical of many systems in which the speed information and thus the measurement of crankshaft position is obtained from the last revolution of the crankshaft, i.e. the Walh crankshaft position transducer outputs only a single crankshaft pulse permitting the reset of its counter only once per revolution of the crankshaft. Thus at relatively low speeds in the order of 600 RPM, where engine control in terms of pollution and efficient operation are the most difficult, the resolution of the position data is in the order of about two cycles per second.
Thus, it has been found difficult to meet the increasingly difficult pollution controls and fuel efficiency standards as set by the U.S. Government by employing control techniques, whether mechanical or electronic, that set the spark advance angle .theta.a as a function of a single parameter such as engine speed or load (as indicated by the manifold vacuum). To achieve these goals, increasingly complex closed loop controls have been developed which employ a microprocessor for the adjustment of the spark advance angle .theta.a, the A/F ratio, the exhaust gas recirculation, etc. Illustratively, such microprocessor implemented ignition control systems employ a crystal oscillator to produce a train of accurate clock pulses that are gated to a counter, one train of pulse for each revolution of the engine crankshaft or distributor. The number of pulses accumulated in the counter at the end of an interval is thus proportional to the period of the crankshaft revolution or inversely proportional to crankshaft speed fs. The microprocessor is programmed to calculate crankshaft speed and uses this value to obtain an indication of crankshaft position of at least a specified angle of interest of the crankshaft rotation. However, such a measurement of engine or crankshaft speed as based upon a single pulse train per revolution is inadequate to achieve the desired degree of control, as contemplated by this invention. More specifically, the prior art computes engine speed based on the last cycle of the crankshaft and, if a change of crankshaft speed has occurred as would be expected during engine acceleration, the value of crankshaft speed derived as by integrating the applied pulses proportional to speed would be in error by an amount portional to the degree of acceleration or deceleration. Further, it is apparent that if the indication of crankshaft speed is based upon a single pulse train per revolution, any speed perturbations due to the firing of the cylinders, whereby the crankshaft alternately accelerates and decelerates during the course of a single revolution of the crankshaft, can not be taken into account by such control systems. Cylinder firing perturbations may cause the instantaneous rotational speed to vary as much plus or minus 60 RPM; at low or idling speeds in the order of 600 RPM, such perturbations may account for an instantaneous speed error of approximately 10%. Thus, such microprocessor implemented ignition control systems could not be used to set the spark advance angle .theta.a very accurately at low speeds. The magnitude of the resulting errors in ignition control are thus the greatest at idle and/or relatively slow engine speeds and for engines with fewer cylinders. Typically, such computer implemented systems provide a "retard" error when the engine is accelerated and an "advance" error when the engine is being decelerated; both such errors degrade engine performance and contribute unfavorably to the emission of pollutants.
Further, the computer implemented ignition control systems employ a read-only-memory (ROM) that is used as a look-up table of spark advance angles, whereby values of engine speed are used to address corresponding values of the spark advance angle .theta.a. The stored values of .theta.a are determined empirically for each engine to be controlled. Obviously, if the addressing value of engine speed is inaccurate, then the resultant value of the spark advance angle .theta.a is in error. As a result, most ignition control systems, even those employing microprocessors, unduly retard the spark advance angle .theta.a within the "EPA Pocket" of engine speeds, thus resulting in poor engine performance.